The application of antireflective coatings to substrates such as solar cells and glass, for example, has heretofore been done by applying a titanium film by vacuum deposition, sputtering and the like and heating to oxidize the film. These processes are expensive and unsuitable for continuous mass production techniques. Recently, more direct methods have been disclosed for applying such coatings by dipping in a solution, or, in the case of small substrates, by spinning on a solution, and then baking or curing to oxidize the film. These methods, which reduce the cost greatly from methods which require a vacuum chamber and expensive equipment, are essentially batch type processes. A spray process, whereby sheets of glass or solar cells can be continuously and controllably sprayed with a titanium compound, would be much less expensive and amenable to a continuous, mass production line operation.
However, the application of an antireflection coating for solar cells, for example, has several very stringent requirements; the refractive index of the film must be in the range of about 2.0 to 2.2; a uniform film of about 700 angstroms thickness must adhere strongly to the substrate; and the film must be stable on prolonged exposure to strong sunlight.
In order to prepare a solution for spray application, it must be made of miscible components; the solution must thoroughly and uniformly wet the surface of the substrate; it must flow uniformly across the substrate surface and coalesce, to form a thin, continuous, and homogeneous liquid film of uniform thickness on the substrate; it must have a viscosity compatible with the available orifice size of the spray nozzle and pressure range of the equipment employed; and the solution should be non-toxic and low in cost. In addition, after oxidation of the film, the final coating must have good adhesion and good integrity to the substrate, must be reasonably uniform in thickness over the entire surface of the substrate, be free of haze and gross structural imperfections, be stable and have the required thickness and refractive index. Presently known solutions for spinning or dipping titanium compounds onto a substrate cannot be employed successfully in spray equipment because they do not spread evenly across the substrate surface, thus building up or depleting areas, particularly on the periphery of the substrate or along elevated lines, such as metallization grids; because they may form turbid solutions; or because the resultant titanium oxide films are non-uniform or hazy.